Field of the Invention
The present invention relates to an electrophotographic image forming apparatus, for example, a digital copying machine, and to a correction method for the image forming apparatus.
Description of the Related Art
In an electrophotographic image forming apparatus, for example, a digital copying machine, lighting control is performed with respect to a laser light source in accordance with an image signal to form an electromagnetic latent image on a photosensitive member, and an image is formed on a recording material through a developing step, a transfer step, and a fixing step. Laser light radiated to the photosensitive member is deflected in a longitudinal direction (hereinafter referred to as “main scanning direction”) of the photosensitive member with rotation of a rotary polygon mirror to scan the photosensitive member. Moreover, with the rotation of the photosensitive member, the scanning is performed also in a direction orthogonal to the main scanning direction (hereinafter referred to as “sub-scanning direction”). As a result, a two-dimensional latent image is formed. Moreover, the laser light deflected with the rotation of the rotary polygon mirror is radiated to the photosensitive member via an fθ lens so that the laser light has a uniform optical path length and angle of incidence in the longitudinal direction of the photosensitive member.
Meanwhile, in Japanese Patent Application Laid-Open No. 2004-338280, there is disclosed, as an optical configuration without an fθ lens for reduction of cost, a method of performing magnification correction entirely with electric correction. In Japanese Patent Application Laid-Open No. 2004-338280, there is disclosed a method of performing the magnification correction by dividing a photosensitive member into predetermined areas in a main scanning direction, and modulating a frequency of a pixel clock depending on a magnification of each area. In a scanning optical system without an fθ lens, a distance from a rotary polygon mirror to the photosensitive member is increased from a center portion toward an end portion of the photosensitive member, and hence a scanning speed at the end portion is higher than the scanning speed at the center portion. As a result, an image formed in the end portion is elongated as compared to an image formed in the center portion. FIG. 13A is a graph for showing a correspondence between a position in the main scanning direction and a magnification of an image to be formed. The vertical axis indicates the magnification of the image to be formed, and a magnification equal to 1 (1×) is a magnification obtained when an image is formed at a center position in the main scanning direction. The horizontal axis indicates a position in the main scanning direction, and 0 mm indicates the center position in the main scanning direction. With the electric correction unit as in Japanese Patent Application Laid-Open No. 2004-338280, even in an optical system in which pixels have different magnifications as in FIG. 13A, the pixels may be corrected to original sizes by assigning reciprocals of the magnifications of the respective pixels to correction magnifications.
There has been known an electric correction unit which employs a magnification correction method in which digital pulse width modulation (PWM) is used to perform processing for each pixel (hereinafter referred to as “divided pixel”) obtained by dividing one pixel in the main scanning direction. In Japanese Patent Application Laid-Open No. 2013-22913, there is disclosed a method in which the magnification correction method is used to vary the magnification by reproducing and inserting the divided pixels instead of modulating the frequency of the pixel clock. For example, with a division number N of the one pixel being 24 (the number of divided pixels being 24), and with a magnification M (M×) in the main scanning direction being in a range of from 1× to 1.3×, the magnification M is changed smoothly along positions in the main scanning direction. In this case, when D divided pixels are selected from among the N divided pixels, and the selected D divided pixels are reproduced and inserted, the new number of divided pixels after reproduction of the divided pixels is (N+D), and a pixel size is (N+D)/N=M×. Therefore, based on (24+0)/24=1× and (24+8)/24=1.3×, the number of divided pixels D to be reproduced and inserted may be selected in a range of from 0 to 8 to obtain a desired varied magnification for each pixel. The obtained magnifications are sparse, but an intermediate magnification may be expressed through combination of different division numbers for local regions.
As described above, with the characteristic of the scanning speed of the scanning optical system without an fθ lens, a magnification to be corrected is determined depending not on a position in the sub-scanning direction but on the position in the main scanning direction. Therefore, when the number of divided pixels is corrected for each main scanning depending on the position in the main scanning direction, pixels having the same division number are arranged at the same position in the main scanning direction as seen from the sub-scanning direction. In a case where pixels having different division numbers are periodically arranged under the state in which the pixels having the same division number are arranged as seen from the sub-scanning direction, when an image pattern that is periodic in the main scanning direction is input to perform the magnification correction, there is a problem in that periods interfere with each other to generate moire. FIG. 13B, FIG. 13C, and FIG. 13D are diagrams for illustrating an example in which moire is generated. In FIG. 13B, FIG. 13C, and FIG. 13D, each square represents a pixel, the horizontal direction indicates the main scanning direction, and the vertical direction indicates the sub-scanning direction. In FIG. 13B, FIG. 13C, and FIG. 13D, a positional relationship of pixels is the same. Moreover, a number in each square of FIG. 13B represents the division number of the pixel (number of divided pixels). In FIG. 13B, there is illustrated an example in which pixels having the same division number are arranged in the sub-scanning direction, and in which pixels having different division numbers are periodically arranged in the main scanning direction. In FIG. 13C, there is illustrated an example of an image pattern of black and white vertical stripes that are periodic in the main scanning direction. FIG. 13D is a diagram for illustrating an image pattern obtained by performing magnification correction of the image pattern of FIG. 13C in accordance with the division numbers of FIG. 13B. In FIG. 13D, the division number of pixels in the vertical direction with the triangle (Δ) mark at the bottom is 25, and the division number of pixels in the vertical direction without the triangle (Δ) mark is 24. In FIG. 13D, there is illustrated how the periods interfere with each other to cause unevenness in widths of black lines and white lines in the vertical direction.